latter form more massive elevations, and are less fractured. Water tapped within the areas of those rocks is therefore generally less salt, and in higher portions even potable.
The general rest level of the subterranean waters within the principal hydrographic basins does not greatly vary, but gradual rises from south-east towards north-west are apparent.
A general subterranean drainage, if existent, would tend accordingly south-eastwards.
The heights above sea of most of the great salt lakes, as Lake Cowan, Lake Lefroy, Lake Barlee, Lake Deborah, Lake Austin, and that of the lakes to the east of Lake Barlee are between 900 and 1,100 feet. Lake Cowan approaches most the former, and Lake Austin the latter height.
The decomposition, and with it the permeability of the gneissic rocks, extends locally down to depths of 200 and more feet from the surface. Within the schistose greenstones it is more general and often attains depths of more than 300 feet.
The subterranean conduits extend chiefly along lodes and fissures The water struck in many of the numerous mining shafts is percolating, and the seeming water level may be many feet above the general rest level of the hydrostatic basin, within which such shafts are sunk.
Some of the subterranean reservoirs contain tolerably potable water. Generally, currents of percolating waters from higher levels pass these reservoirs on their journey towards the general collecting basin. The permeable rocks within the reservoirs are already leached, and the water which drains into them is still fresh. Such reservoirs are numerous within the interior gold region, although they are not always to be found in the immediate vicinity of mining camps.
VI.—GOLD DEPOSITS.
(a) Primary Gold Deposits.
The gold deposits within the interior gold region occur in stretches of country, the longitudinal axes of which strike in various directions, but in travelling over this region the prevalence of a general north-westerly tend can hardly remain unobserved.
It is not a matter of accident, but of cause and effect, that most of the gold deposits are situated in the vicinity of saline depressions and so-called salt lakes; they usually occur along lines of fracture and subsidence.
The ejection of huge masses of aqueo-igneous magma to the surface has necessarily left large cavities underneath. The subsequent fracture and subsidence of portions of earth-crust, which have formed the roofs of such cavities, have caused the depressions which appear now as saline flats and salt lakes.
The lines of fracture present generally systems of faults and fissures, along which one portion of the broken country has remained in a relatively higher position, whereas the other has subsided.
Gneissic granites rise occasionally to the surface forming contacts with palæozic greenstones; they are generally part of the relative higher portion of country along the line of fracture.
Some of the fissures have served as vents for igneous magma, which ultimately became solidified within. They now form dykes; others were filled with quartz simply by secretion; others again became vents for solfatara solutions, which have risen under steam and gas pressure. Those solutions have filled their fissures chiefly with quartz, with auriferous quartz and with metalliferous minerals, which usually are associated with gold.
During subsequent movements in the earth-crust, complete quartz lodes, as well as dykes, were broken through, and sometimes dislocated by newly-formed fissures, and in some instances the original lode or dyke-fissure was partly reopened.
Fractured portions of such broken reefs and dykes gave easy access to circulating mineral solutions. If those solutions were derived from deep-seated solfatara action, and were auriferous, these conditions favoured the formation of rich gold shoots and auriferous ore columns in otherwise poor or barren quartz reefs or dykes. Along contacts between gneissic granites and greenstones, odes or reefsare usually found to occur in both formations, and occasionally passing from one into the other.
We may suppose with many modern geologists, that the rocks of the archæan era are the original seat of the gold. Rivers crossing formations consisting of such rocks generally produce gold sand deposits, although no reefs or lodes occur in their vicinity; such deposits may be technically worthless, in consequence of their small yield, but they give support to the supposition, and make it appear very probable that gold occurs in a metallic state, finely distributed in the archæan rocks. In accordance with that supposition, it becomes possible to explain the extraction and transportation of gold, and also the formation of auriferous deposits by natural agencies, which have left their traces of action behind.
Archæan gneissic granites underlie the interior gold region and the material of which the palæozoic greenstones were formed has risen to the surface or to higher levels (in a state of aqueo-igneous fusion), through fissures which occurred in the former. Such magma ejections have taken place along great lengths of those fissures, and apparently were accompanied by overheated steam and gas explosions (no matter if steam and gas were originally contained in a compressed state in the pyrosphere, or were developed under access of oceanic waters, which through fissures have reached the fiery interior of the earth).
When such fissure-ejections ceased to occur frequently, the largest portions of the fissures became closed by solidified magma, and the igneous actions within this region became more like those which can be observed in recent volcanism.
Crater eruptions have succeeded fissure emanations, and after the cessation of the latter, fumarales, solfataras, and mud volcanoes were the last manifestations of igneous agencies within this portion of the earth's surface.
It appears probable that the active magma ejections have occurred during the later periods of the palæozoic era, and that commencing in the western parts of the present gold region, they have proceeded with a simultaneous recession of the palæozoic ocean in an easterly and south-easterly direction.
At the beginning of the mesozoic era a general elevation of this region above sea was completed, and the formation of most of the primary gold deposits has probably taken place during the early periods of that era.
Doubtlessly the formation of lodes commenced with the first fracture of the archæan lythosphere, but the formation of most of the primary gold deposits within this region is due to hydrothermal gold emanations. The latter have succeeded the greenstone eruptions for which the present dyke fissures have served as vents.
The effects of solfatara action on rocks can be observed in volcanic regions, and it is obvious that such action must be much more intense when deep-seated. Its highest degree is hydrothermal fusion.
